Why does water form a sphere when it drops?

Water minimizes its surface area by forming a sphere because surface molecules experience unbalanced attractive forces. Interior molecules are pulled equally in all directions by cohesive forces, but surface molecules are attracted only downward. This creates surface tension, causing the liquid to contract into the shape with the lowest surface-area-to-volume ratio, which is a sphere.

What causes water to climb up the inside of a glass tube?

Water climbs glass tubes through capillary action, driven by adhesive forces between water and glass that exceed cohesive forces within water. Water molecules spread along the tube's surfaces via adhesion, drawing the rest of the liquid upward by cohesion. This continues until gravity balances the adhesive and cohesive forces, with narrower tubes allowing water to rise higher.

Why does mercury form a different meniscus shape than water in a tube?

Mercury forms a convex meniscus because its cohesive forces between mercury atoms far exceed adhesive forces with glass. In contrast, water forms a concave meniscus because adhesive forces between water and glass are stronger than cohesive forces within water. The relative strength of these intermolecular forces determines whether the liquid wets or beads on a surface.

Why is glycerol more viscous than methanol?

Glycerol is more viscous than methanol because it has three hydroxyl groups that form more hydrogen bonds per molecule, creating stronger intermolecular attractions. Although both liquids form hydrogen bonds, glycerol's additional bonding sites result in greater resistance to flow. Stronger intermolecular forces make molecules move less freely, increasing viscosity.

How does temperature affect a liquid's viscosity?

Increased temperature decreases viscosity because thermal energy weakens intermolecular forces, allowing molecules to move more freely. As kinetic energy increases, molecules can overcome the attractive forces holding them together more easily. This enables liquids to flow faster at higher temperatures, reducing their resistance to flow.

Why do longer hydrocarbon molecules have higher viscosity?

Longer hydrocarbon molecules have higher viscosity because they interact over greater surface areas and entangle more frequently, strengthening dispersion forces between them. These increased intermolecular interactions create greater resistance to flow. Hydrocarbons with increasing molar masses and lengths experience stronger attractions, making them more viscous than shorter chain hydrocarbons.

How do cohesive and adhesive forces differ in their effects on liquids?

Cohesive forces are attractions between identical molecules within a liquid, while adhesive forces are attractions between different molecules, such as water and glass. Cohesive forces cause liquids to minimize surface area and resist spreading. Adhesive forces determine whether a liquid wets a surface or beads up, depending on their relative strength compared to cohesive forces.

11.5

Surface Tension, Capillary Action, and Viscosity

29.6K views02:57 min

Ever wondered why a yen coin, when carefully placed on water, floats?

Most properties of liquids are dictated by intermolecular forces. The attractive forces between similar molecules in a substance are called cohesive forces.

In water, the cohesive forces pull interior molecules equally in all directions, resulting in zero net force on average, while surface molecules experience only a downward pull, which causes the molecules to pack closely together.

The interior molecules are more energetically stable than surface molecules because they experience more cohesion, which lowers their potential energy.

Fluids, therefore, try to reduce their potential energy by minimizing the surface area, creating a surface under tension that behaves like an elastic membrane.

The energy needed to increase a liquid’s surface area by a unit amount is called its surface tension, which is often measured in joules per meter-squared.

Adhesive forces, in contrast, exist between dissimilar molecules, such as water and glass.

Upon placing a narrow capillary tube in water, the water molecules spread along the tube's surfaces via adhesion, increasing the liquid's surface area and drawing the rest of the liquid with it by cohesion. The liquid in the capillary rises until gravity overcomes the adhesive and cohesive forces. This phenomenon is called capillary action.

For water, the adhesive forces between water and glass are stronger than the cohesive forces, resulting in a concave, or inward-curving, meniscus. For mercury, the cohesive forces outweigh the adhesive forces, resulting in a convex meniscus.

Viscosity expresses a liquid's resistance to flow and is often measured in poise, or grams per centimeter-second,

Consider methanol and glycerol. Although both form hydrogen bonds, glycerol has three –OH groups instead of one and forms more hydrogen bonds per molecule. This results in a stronger attraction between glycerol molecules, making glycerol more viscous than methanol.

Viscosity is influenced by molecular shapes. Hydrocarbons of increasing molar masses and lengths interact over greater areas and entangle more frequently, resulting in stronger dispersion forces and higher viscosities.

Lastly, viscosity depends on temperature. Increased thermal energy weakens intermolecular forces, corresponding to molecules moving more freely and liquids thus flowing faster at higher temperatures.